feat: sidebar unread badges, notification deep-link, bell dedup (0.4.90)

[kwalus]

Summary

• Add left-rail sidebar unread badges for Messages, Channels, and Social Feed with periodic polling refresh
• Add durable per-user feed-view acknowledgement so feed unread count reflects real new activity
• Fix notification bell deep-links to navigate to exact target messages via server-side focused context window
• Add semantic dedupe for bell events so mention+channel_message for the same source only count once
• Add container-aware focus scrolling in channels for reliable message targeting

Test plan

• pytest tests/test_sidebar_attention_summary.py tests/test_feed_unread_state.py tests/test_frontend_regressions.py tests/test_channel_message_route_regressions.py -q
• python3 scripts/check_jinja_templates.py